1. Field of the Invention
The present invention relates to a process for manufacturing corrosion resistant copper tubes with the interior surface tinned which are used as water supply tubes, hot water supply tubes, tubes in heat exchangers, and the like, and, more particularly, to a process for manufacturing interior tinned long copper tubes in which the plated tin films have no defects and are highly corrosion resistant.
2. Description of the Background Art
Plating tin films inside copper tubes which are used as water supply tubes, hot water supply tubes, tubes in heat exchangers, and the like, to improve corrosion resistance and to prevent elution of copper ions from the tubes, is well known in the art. In particular, a process for causing a substitution type electroless tinning solution to flow through the inside of a copper tube has been proposed as a method for forming tinned films inside a long coil of copper tube (Japanese Patent Application Laid-open No. 45282/1992)
This method is very simple and efficient for processing a long coil of copper tube and produces thin tinned films with superior adhesive properties. However, when the products are used for a long period of time, elution of copper ions from the products due to wear and peeling of tinned films is detected. A method of subjecting tinned films to an oxidization treatment with hot water or steam to improve corrosion resistance has also been proposed (Japanese Patent Application Laid-open No. 99180/1992). However, the tinned film formed by this method also produces pitted corrosion when used under severe conditions. Generally, the corrosion resistance of a long copper tube tends to be insufficient, because it is difficult to form a uniform tinned film over the entire surface of a long copper tube. Improvement in the corrosion resistance in such a long copper tube has therefore been desired.
Another process which has been proposed for forming uniform plated films without defects and for improving pitted corrosion resistance or the like comprises controlling the ratio of the copper ion concentration and the tin ion concentration (copper ion concentration/tin ion concentration) in the plating bath to 0.7 to less, when tinning is carried out by dipping a copper plate in a plating bath or by circulating a plating solution inside a short copper tube with a length of several meters or less (Japanese Patent Application Laid-open No. 339741/1993). In this method, a uniform plating filmis produced by controlling the performance of the plating solution by supplying a fresh plating solution or adding chemicals such as a tin salt when the performance of the plating solution decreases.
However, because it takes a long time for the plating solution to be circulated in a long coil of copper tube with a length of from about thousand meters to a thousand and several hundred meters, it is unavoidable that the properties of the plating solution introduced from one end of the tube changes when the solution flows from the other end of the tube. Specifically, as a result of the reaction represented by the formula, EQU Sn.sup.2+ +2Cu=Sn+2Cu.sup.+
which occurs while the plating solution is circulated inside the copper tube, tin (II) ions are consumed and copper ions gradually accumulate, causing the quality of the tinned film to deteriorate on the side from which the plating solution flows. The longer the copper tube, the more remarkable this tendency. Therefore, control of the plating solution for tinning a long copper tube should be performed from a different viewpoint from the tinning process for a short copper tube.
A plating solution containing stannous sulfate and the like is circulated when substitution-type electroless tin plating is performed inside a long copper tube at a plating temperature usually of 60 to 70.degree. C. If the temperature is 40.degree. C. or lower, for instance, a thick plated film is produced only with difficulty due to a slow rate of deposition of the plating film material. In addition, the size of the deposited tin particles fluctuates, resulting in production of a number of pinholes. It is thus difficult to obtain a uniform plated film with sufficient corrosion resistance. However, deposition of plating film material is too fast when a long copper tube is tinned at a high temperature, resulting in accelerated consumption of tin (II) ions and accumulation of copper ions. This results in a decrease in the plated film thickness over the inner surface of copper tube, increase in the number of pinholes, and decrease in the adhesion strength of the film on the side from which the plating solution flows. Therefore, the maximum length of a copper tube which can be adequately tinned is about 200 m (9 m.sup.2).
The number of pinholes in plated films can be reduced by simply increasing the film thickness to about 2 mm or more. Increasing the film thickness, however, is accompanied by an increase in the amount of tin (II) ion consumed from the plating solution. This involves an increase in the cost for chemicals. In addition, production of a thick film requires a longer plating time, also resulting in increased plating costs.
Coating tin inside a copper tube by electroplating rather than electroless tin plating may be one method for preventing formation of pinholes. Because electroplating produces tinned films with less pinholes, this method is effective in preventing pinhole production. However, to cover the whole length of copper tube with a uniform tin film by electroplating requires provision of a pair of electrodes in the tube. These electrodes must be installed so that no part thereof comes into contact with the tube wall. This is a difficult task to perform, particularly when the tube which is to be tinned is a small diameter coiled tube. Consequently, development of an electroless tin plating process with decreased pinhole production has been desired.